Department for Transport says extreme wet weather may have led to the lower figures. Photograph: Matt Cardy/Getty Images

The number of deaths on the roads this spring fell sharply, provisional government figures showed today. A total of 400 people were killed in reported road accidents in Britain in the period April to June – an 18% reduction on the figure of 487 in the same period last year.

The number of serious and slight injuries on the roads also decreased, with the Department for Transport saying the extreme wet weather might have led to the lower figures.

The provisional figures showed that the number of people killed or seriously injured (KSIs) fell 6% to 5,970 in spring this year. Slight injuries were down 7% at 41,910, while total casualties (killed, seriously injured and slightly injured) fell 7% to 47,880.

Child KSIs this spring fell 14% to 610, while total child casualties in April to June this year were down 16% – the largest fall in more than 20 years.

The DfT said: "In general, single-quarter comparisons should be treated with caution as the data are more variable, and more subject to distortion by short-term factors such as unexpected weather.

"In this context, it should be noted that Q2 2012 (April to June) saw extremely wet weather across England (where rainfall levels were more than double the 1981/2010 average in both April and June)."

The DfT went on: "This wet weather is likely to have reduced levels of pedal cycle and motorcycle traffic in particular. Provisional traffic statistics show a fall in overall traffic of 1.1% between 2011 Q2 and 2012 Q2, with a much larger fall in the other motor vehicles category (which incorporates motorcycles).

"There is also evidence that the wet weather may have affected road-user behaviour through reducing traffic speeds. Compared to the same months in 2011, observed average speeds on the local A-road network were 4% lower in April 2012 and 1% lower in June 2012."

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